Effects Of Elevated Atmospheric CO₂ And Temperature,Nitrogen Addition On The Photosynthetic Characteristics And Biomass Distribution Of Hybrid Poplar Saplings

The impact of global changes on terrestrial ecosystems is extensive and profound.Factors such as the elevated of atmospheric CO₂ and temperature and nitrogen addition can significantly affect the growth and physiological and ecological processes of trees,thus affecting the productivity and long-term health of forest ecosystems.Poplar（Populus spp.）is a model species in model tree for the study of woody plants.It is great significance for vegetation construction and sustainable management in semi-arid areas of northern China to clarify the influence of major factors of global change on poplar growth and physiological and ecological processes.In this paper,the effects and interactions of high CO₂ concentration and temperature rise on photosynthetic characteristics and dry matter distribution of functional leaves of（P.× popularis‘35-44’）cuttings were studied in open top artificial climate chamber（OTC）.The effects of nitrogen addition intensity on growth,leaf gas exchange,biomass distribution and stem embolism in early spring of the second year of（P.× euramericana ‘Zhongjin’）cuttings were studied.The main research results are as follows:Photosynthetic capacity of functional leaves of populus was enhanced with the elevated of atmospheric CO₂ concentration,energy distribution ratio of photoreaction center to photosynthesis was significantly increased,while electron transfer efficiency and carboxylation efficiency were not significantly changed.Stomatal conductance and stomatal density of leaves were significantly reduced,transpiration rate was weakened,and instantaneous WUE was improved.The increase of photosynthetic capacity increased the accumulation of biomass per plant,and the mass poplar distributed more biomass to underground roots.The stomatal density of the functional leaves of poplar seedlings increased significantly to reduce the leaf temperature in elevated temperature.The gas exchange parameters of the leaves did not change significantly,but the rate of carboxylation and photorespiration increased significantly,and the photorespiration rate may be the main factor limiting the growth of the masses.Under the interactive condition,leaf photosynthetic nitrogen utilization efficiency（PNUE）was significantly increased,and the warming treatment partially offset the promotion effect of elevated CO₂ concentration on photosynthetic characteristics and growth of poplar.Nitrogen addition significantly increased leaf area,chlorophyll content and photosynthetic capacity,stomatal conductance and transpiration rate of Poplar seedlings,and improved instantaneous water use efficiency.However,the addition of 9g/plant significantly reduced the photosynthetic capacity of leaves.The growth of the first year cuttings and the proportion of stem biomass were significantly increased with different nitrogen content.With the increase of nitrogen content,the shrinkage of Populus ciclosa in the second spring increased significantly,and catheter embolization was the direct cause of the shrinkage.In the second year,the proportion of leaf and stem biomass increased gradually.In summary,the increase of atmospheric CO₂ concentration can improve the photosynthetic characteristics and water use efficiency of poplar leaves in the long term,dry matter is more distributed in the underground root system,while the growth of poplar leaves is significantly reduced by the increase of temperature,and the promotion effect of the increase of CO₂ concentration is weakened by the increase of temperature under the interaction.Nitrogen addition was beneficial to the growth of Poplar seedlings in the first year,but increased the incidence of stem and catheter embolization in the second year and led to the phenomenon of withered.These results are helpful to reveal the response mechanism of trees to global change factors and provide a theoretical basis for the sustainable development of poplar artificial forests in semi-arid areas of northern China.